DE 101 42 971 A1 describes a power semiconductor module containing a ceramic substrate which is fitted with semiconductor components. The semiconductor components are contact-connected to interconnects on the top side of the substrate, said interconnects leading, inter alia, to contact pins for externally connecting the module.
The underside of the substrate is copper-coated and functions as a heat-dissipating contact area in order to be able to dissipate power losses—which occur in the form of heat—to a heat sink or a cooling element during operation of the semiconductor components.
The substrate is surrounded by a (plastic) module housing and is pressed onto the heat sink by means of screw connections, for example. Continuous area contact (which is free of air inclusions) between the underside of the substrate and the heat sink is desirable for optimum heat dissipation. However, as a result of the high-temperature production process, the substrate is subject to internal mechanical stresses on account of different coefficients of expansion of the conductor (e.g. copper) and the substrate (e.g. ceramic). These mechanical stresses may also be intensified by subsequent production steps (e.g. soldering operations). As a result, curvature of the substrate often cannot be avoided, with the result that the contact area for the heat sink is not planar.
In order to counteract this problem, external or housing-internal pressure stamps are provided in the module disclosed in DE 101 42 971 A1, said pressure stamps transmitting externally applied pressure to various points or locations of the substrate in order to press the substrate, as uniformly as possible, onto the heat sink. This is assisted by the substrate being advantageously subdivided into a plurality of cohesive substrate regions which can be moved relative to one another to a limited extent.
However, substrates having a relatively large areal extent, in particular, are extremely sensitive to the individual contact pressure force that is acceptable at the individual locations. DE 101 49 886 A1 therefore followed the route of connecting the contact pressure elements to the housing in a resilient manner in order to be able to set the pressing-on pressure in an improved manner using the spring forces.
In this case, however, it proves to be problematic, in the case of a relatively large number of individual contact pressure elements, to use the latter to uniformly apply pressure, particularly in the central inner region of the substrate. If an excessively high external force is applied, this leads to locally excessive pressing-on with excessive pressure and to the risk of the brittle ceramic breaking. If, in contrast, the contact pressure force is selected to be excessively low overall in order to avoid damage to the substrate, regions remain which are not in contact with the heat sink. In the case of an external force which is introduced via the module housing, for example, component tolerances additionally have a disadvantageous effect since they may lead to pressure being applied to the individual substrate locations to differing degrees.